Both gaseous and liquid fuels are burned in gas turbine engines used to drive electrical generators and mechanical equipment. Liquid fuels are burned exclusively in gas turbine engines used for aeronautical applications. Over the past ten years, the lean-premixed (LP) combustor has been developed and is now the accepted device for burning natural gas in gas turbine engines. It is economical and meets most environmental regulations.
Although many gas turbines run predominately on natural gas, their combustor must have the ability to cleanly and efficiently burn oil or other liquid fuels during the coldest part of the year, since nearly all sites are on interruptible natural gas service. Additionally, for some sites, liquid fuel is the fuel of choice for year-round use. And, as mentioned above, liquid fuel is the only fuel consumed in aeronautical gas turbine engines.
Liquid fuel combustors for gas turbines are not as advanced as LP combustors for natural gas. Consequently, pollutant emissions are significantly greater for liquid fuel firing than for gaseous fuel firing. Most liquid fuel combustors are non-premixed (the fuel and air enter the combustor separately). In order to overcome the problem of high emissions, the gas turbine industry is developing lean, prevaporized, and premixed (LPP) combustors. Thus, the device that provides the prevaporized-premixed fuel and air mixture to the flame (combustor) is very important.
Number 2 diesel fuel is the preferred liquid fuel for may gas turbine engines. Kerosene and JP fuels are used in aero engines. In addition, under certain circumstances, light distillates (i.e. naphtha) and/or heavy distillates or residual fuel oil are used as the fuel. Fuel preference depends on availability and on economical and geo-political factors. For example, in Asia, a mixture of both LP and LPP combustion turbines is in heavy demand, but a large variation in fuel availability and demand exists.
Environmental impact from the use of gas turbines is considered to be one of the lowest compared to other combustion devices. Nevertheless, environmental awareness has prompted regulatory agencies throughout the world to place increasingly stringent requirements on the reduction of both gaseous and particulate pollutant emissions from gas turbines.
Pollutant emissions from the combustion of liquid and gaseous fuels in gas turbines mainly consist of nitrogen oxides (NO.sub.x), carbon monoxide (CO), and particulates (e.g. soot and sulfate particulates). Emissions from LP combustors running on natural gas are generally low and are controlled by the lean-premixed combustion process. Emissions under 10 ppmv (parts per million by volume) are obtained for some LP combustion gas turbines. On the other hand, for liquid fuel fired gas turbines, water or steam injection is required to reduce the NO.sub.x emissions to about 40 ppmv, a practice many wish to avoid because of inconvenience, cost of providing very clean water to the engine, and degradation of the engine. The state of the U.S. technology on LPP combustors is such that emissions under 70 ppmv of NO.sub.x are difficult to obtain.
In an LPP combustion system, a prevaporization process is employed to vaporize and premix the fuel and air before the fuel and air enter the combustor. In actuality, current LPP gas turbine designs can only partially prevaporize the liquid fuel before it is introduced into the combustor. Because the fuel is only partially prevaporized, it cannot be completely premixed at the molecular level with the air prior to combustion. Consequently, flame temperature and NO.sub.x formation rates are higher than for the case of completely prevaporized-premixed combustion. For this particular reason, water or steam is injected into the combustor primary zone to reduce and control the formation of the oxides of nitrogen. As noted above, the additional requirement of a water or steam injection system increases the capital, operating, and maintenance costs of the LPP gas turbines.
Prior art has attempted to address prevaporizer-premixers for combustors that reduce noxious emissions, such as Richardson, U.S. Pat. No. 5,647,538, granted Jul. 15, 1997, and entitled "Gas Turbine Engine Fuel Injection Apparatus"; Beebe et al., U.S. Pat. No. 5,295,352, granted Mar. 22, 1994, and entitled "Dual Fuel Injector With Premixing Capability For Low Emissions Combustion"; Teets, U.S. Pat. No. 4,429,527, granted Feb. 7, 1984, and entitled "Turbine Engine With Combustor Premix System"; Hammond, Jr. et al., U.S. Pat. No. 5,3958,416, granted May 25, 1976, and entitled "Combustion Apparatus"; and Verdouw, U.S. Pat. No. 3,925,002, granted Dec. 9, 1975, and entitled "Air Preheating Combustion Apparatus." However, all of these patents are directed to single stage premixers, meaning there are not two sources of air temperatures, to more fully prevaporize and premix the fuel for optimum performance and reduced pollutant emissions. Although Teets addresses two "stages," it is not a true two stage prevaporizer-premixer as there is no addition of higher temperature air to fully prevaporize and premix the fuel and air.
Additionally, the prior art does not address the problem of coking as the liquid fuel is sprayed or otherwise discharged onto the hot metal surface of the premixer during the atomization and vaporization process. Coking, which is the oxidative pyrolysis of the parent fuel molecule into smaller organic compounds and its eventual transformation into solid carbon particles, is undesirable since it leads to deposition of solid carbon particles on hot surfaces, which eventually leads to flow disruption. The rate of deposition (i.e. coke formation) is dependent on the (wall) surface temperature, fuel temperature, pressure, and fuel type. In particular, it is strongly influenced by (wall) surface and fuel temperature. The range in which coking can be a problem is 400 K and above, which is the temperature range at which most premixers operate.
An object of the present invention is to provide a two stage prevaporizing and premixing process without the need to add water or steam to reduce pollutant emissions. Another object of the present invention is to mitigate coking by keeping the liquid fuel away from hot surfaces. Another object of the present invention is to mitigate flashback (that is the propagation of the flame back into the premixer).